Engineering Geological and Stress Induced Instability Evaluation Along Middagselvfjell Railway Tunnel

Abstract

Ofotbanen is over 100 years old and was originally constructed for transporting ore from iron mine in Kiruna (northern Sweden) to Narvik harbour. The traffic today is composed of ore trains, freight trains and passenger trains. In the light of the expected increase in ore production, as well as some increase in freight and passenger transportation, the capacity on the single tracked railway must be increased. Norwegian National Rail Administration suggest increasing the capacity by building a double tracked railway. Where the exsiting track is kept and new track is built, in most cases next to the existing track with exception to few tunnels in geology challenging areas. A single track tunnel through Middagselvfjell is one of the suggested tunnel in the double track on Ofotbanen project. The pre-existing railway around Middagsevlfjell is characterized by choppy curvature and the terrain is characterized by steep hillsides. Hence, engineering geological conditions and stress induced stability assessment of the Middagselvfjell mountain are needed for tunnel project in the area. Middagselvfjell tunnel is planned a 3.8 km long tunnel, with a E-W direction, constructed from Indre Sildvik through the Middagselvfjell towards the Tappelv area.

Stability is an essential factor for an underground excavation. Problems related to stability are generally because of the high stresses surrounding the periphery of the tunnel opening. Failure is a form or measure of stability, failure of rock occurs if induced stresses exceed rock mass strength. Nordland County in Northern Norway has experienced rock failure problems related to high horizontal stresses in the area. Which happened in the Sildvik hydropower plant located only few hundred meters south of the planned Middagselvfjell tunnel.

The thesis covers the engineering geological issues related to excavation of Middagselvfjell tunnel. The study is based on field investigation and laboratory testing, with a focus on method predicting and assessing brittle failure in the tunnel. Numerical, analytical and empirical approaches for instability prediction are applied.

Results show that conditions for tunnel excavations at Middaselvfjell are generally favourable in terms of stability. The rock is generally strong with the estimated Q value around 36 and the intact rock mass strength UCS of 221 MPa. Stress induced instabilities can be expected very anisotropic stress state where the k ratio is greater than three, i.e. the horizontal stress component is 3 times higher than the vertical stress component.